[Montana Man]

Firestation12, a plastic tank filled will experience deformity to some degree. Proper bracing will reduce deformity and prevent malfunction. As a past hydraulic mechanic, I understand the rationale you use to make your point even though Boyle's law generally applies to gasses. Pascale's law speaks to fluids in a confined space. The problem is there isn't enough resistance to the incoming fluid to reach the pressures in your example. If the tank were not vented then yes, the laws of hydraulics physics would apply in a much different manner.

Why is it that many of us fill the FW tank until it overflows and not rupture the tank? Because those tanks are not of the same design and/or mounting of the grey and black tanks. The waste tanks are buckling under their own weight. Not due to being pressureized to thousands of pounds.